Mass spectrometer with particular ion source for analysis of solid bodies



Dec. 14, 1965 w. HANLEIN ETAL 3,223,836

MASS SPECTROMETER WITH PARTICULAR ION SOURCE FOR ANALYSIS OF SOLID BODIES Filed April 27, 1962 United States Patent 3,223,836 MASS SPECTROMETER WITH PARTHIULAR 0N SOURCE FOR ANALYSIS OF SGLHD BODIES Walter I-Iainiein, Eriaugen, and Karl-Georg Giinther,

Nurnberg, Germany, assignors to Siemens-Schuckertwerlre Alrtiengesellschaft, Beriin-Siemensstadt, Germany, a corporation of Germany Filed Apr. 27, 1962, Ser. No. 191,405 Claims priority, application Germany, Apr. 29, 1961, S 73,759 7 Claims. (Cl. 250-419) Our invention relates to methods and devices for analyzing solid bodies on mass spectrometrical principles, and is an improvement over Paul et al. U.S. Patent 2,939,952, from which the following three paragraphs are quoted, with minor changes.

The invention relates to methods of separating or separately detecting charged particles of different specific charges (e/m==ionic charge divided by ionic mass).

Such arrangements may be employed in the usual manner for mass-spectroscopy and isolation of isotopes, further as pressure-gauges for measuring partial pressure of the components of highly thinned gas mixtures, to determine leaks in vacuum apparatus, to analyze traces and to measure small vapor pressure, particularly in gas mixtures. In the past ion separating devices which offer both high accuracy and high intensity measurements have been extremely expensive and complex in view of the physical principles upon which their operation is based.

The invention utilizes an electric field which is periodical in time but which does not serve for measuring a transit time or the velocity of the ions and is characterized in that the potential of the electric field is a quadratic function of the coordinates x, y, z. The most general such potential is defined by the equation:

Here f(t) is an arbitrary periodic function of the time 2. Because of the Laplacian A =0 the constants ,5 and have to satisfy the equation oz+l3='y. When ions are brought into such a field, their equations of motion are differential equations with periodical coefficients, the equations being characterized by having ranges of stable and unstable solutions. Thus, there exist two different kinds of ion paths; either the ions perform oscillations around the center of symmetry of the field, the amplitudes of the oscillations remaining smaller than a certain maximum value (stable paths), or the amplitudes of the oscillations increase extremely rapidly so that, within a very short time, the particles impinge on the field generating electrodes and are thus removed (unstable paths). When the field and the dependence on the time f(t) are given, the specific charge of a particular ion will determine whether it travels along a stable or unstable path. In particular, the stability or instability of the paths is independent of the point of origin and of the direction and magnitude of the initial velocity of the ion. If e/m of the ion lies in a stable range, then all its possible paths are stable and, conversely, if e/m of the ion lies in an unstable range, then all its possible paths are unstable. The positions and widths of the stable ranges of the specific charge can be varied within very wide limits solely by varying the amplitude, frequency and/ or shape of the field creating voltages which determine the function f( The term solid bodies includes composite bodies or substances such as glasses, ceramics and alloys. It is known to use a mass spectrometer having a sector magnet, and in which a crucible for evaporating a specimen of the solid body under investigation is inserted into the ion source, the escaping vapor being ionized and the ions 3,223,836 Patented Dec. 14, 1965 separated in accordance with their respectively different masses. Analyzing devices of this type are intricate in construction and difficult to operate.

An object of our invention is to greatly simplify such equipment and to provide more rapid solid-body analysis.

According to our invention we subject the substance under investigation to cathode sputtering, thereby gasifying and/ or vaporizing it, and we simultaneously or subsequently ionize this gas or vapor phase before subjecting it to analysis in the mass spectrometer proper, particularly in a mass filter. More specifically, the gases or vapors of the solid body or its constituents are produced primarily by bombardment, preferably by ions, the ionized vapors or gases resulting therefrom being separated in accordance with their different masses.

. According to another feature of our invention the cathode sputtering and ionization are preferably effected in the ion source itself. To increase the disintegrating speed, an auxiliary neutral gas, preferably a noble gas, can be added to the electric discharge space, being preferably used in a dosage resulting in optimal discharge conditions. The disintegration rate increases approximately in proportion to the pressure up to pressures of 5-10- mm. Hg maximum. Consequently this value should be taken as a reference point for adjusting the optimal pressure. At pressures above 5 -10 mm. Hg, space charges may be formed; the mean free path then becoming too small, and the extraction of the ions more difficult. The auxiliary gas must be highly purified, and also have a high mass number m, because the rate of disintegration or sputtering increases with increasing mass number. The gas should have a simple mass spectrum, for which reason atomic gases are preferably employed, all noble gases being atomic gases. As a rule, argon, having the mass number 40, can be employed. Where the mass 40 may interfere with the proper indication of the mass filter by obscuring any peaks of mass numbers at or near 40, derived from the substance being investigated, a different auxiliary gas, for example, helium, can be employed.

Applicable for the above-described method is a mass filter based on the principle of Paul, as known from U.S. Patent 2,939,952. This mass filter, for the purposes of the instant invention, can be so constructed that the specimen of the solid body is arranged on a rod-shaped extension of the eathodea so-called rod cathode-directed toward the analyzer portion of the mass filter. Reference is further made to copending U.S. application Serial No. 94,070. The rod cathode may be hollow, and is preferably a capillary tube. The end of the hollow rod cathode is preferably perforated or otherwise designed as a screen for the passage of the substance being investigated, its gases or vapors or ions. This will be described below with reference to FIG. 4. Preferably, at least those parts of the ion source which may become contaminated in the mass-spectrometrical sense by the substance to be investigated are designed to be readily exchangeable. It is preferable, furthermore, to provide a feeder tube which facilitates insertion of the solid body being investigated into the hollow of the rod cathode; the insertion being from outside the outer wall of the measuring cell in which the cat-bode is mounted. If desired, a plurality of such feeder pipes or feeder locations may be provided in a single mass filter. It is also preferable to provide the feeder tube with a lock which, if desired, may be latchable. Where the solid body specimen is available in fine pulverized form it may thus be readily introduced into the hollow of the rod cathode. For example, the powder can be compressed into a small body or may be prepared as a paste, and then introduced. It is preferable to combine all the components of the mass filter, required for the solidbody analysis, with the measuring cell and such accessories as the high-frequency generator, rectifier, directcurrent measuring amplifier, recorder, current supply equipment for connection to a utility line, and the feeder devices for the solid body substances to be investigated. All of these components jointly form a single enclosed unit, preferably of the transportable type. The abovementioned objects and other objects, advantages and features of our invention, will be apparent from, and will be described with reference to the accompanying drawing, said features being set forth with particularity in the claims annexed hereto.

FIG. 1 shows schematically a complete mass-filtering equipment;

FIG. 2 illustrates, in cross section, a cold ion source for the measuring cell of the said equipment, requiring the mass filter to be opened for inserting the specimen to be tested;

FIG. 3 is a sectional view of another embodiment of a cold ion source for a mass filter according to the invention permitting the specimen to be inserted from the outside without opening the mass filter;

FIG. 4 shows in section the screen-type end of a rod cathode applicable in devices according to the invention; and

FIG. 5 is a sectional view of another cold ion source for the purposes of the invention with readily exchangeable components.

In FIG. 1, the measuring cell 1 of the mass filter comprises the ion source 2, supplied with the substance to be investigated, a set of analyzer electrode rods 3, and an ion-current collector 4. These components are mounted within a vacuum-tight vessel 5. Connected to a neck 6 of the vessel is ahigh-vacuum exhaust pump 40 and a prepump 41. The analyzer electrode rods 3 are connected to a high-frequency generator 8 to supply them with electric power of suitable voltage and frequency. The ion current impinging upon collector 4 is amplified by an amplifier 9 and supplied to a recorder 10 or other measuring instrument. Another indicating instrument 10', at the left, permits supervision of the operation of ion source 2.

The envelopes or vessels of the measuring cells may consist of glass or of metal and their general design and performance may be in accordance with details described in the above-mentioned patent, or in the copending U.S. applications of Giinther et al., Serial No. 94,071 now U.S. Patent 3,143,647 and Serial No. 97,244, new U.S. Patent 3,105,899.

FIG. 2 illustrates a cold ion source for measuring appararatus constructed according to the invention, the apparatus 'being provided with a rod cathode 22 which serves for fixing the plasma as well as for mounting the solid test body or specimen P. The ion source comprises a permanent magnet 1' 1 mechanically joined with a potshaped housing 13 which also serves as a magnetizable yoke, forming part of the magnetic circuit energized by magnet 11. The yoke 13 has an outlet opening 114 for the ions. Another outlet or opening 12 of the yoke 13 may serve, for example, for supplying a neutral gas. A cylindrical anode 15 is coaxially adjacent to the pole face 16a of the pole shoe 16, the latter constituting part of the cathode. The anode 15 is connected to the operating 'voltage through a terminal 17 connected to the anode through a lead 17a, the lead passing through the yoke .13, through an insulating bushing 18. The yoke 13 and the parts conductively connected therewith are preferably grounded, through a grounding terminal '19. An insulator 3t) secures the anode 15 in proper position and electrically insulates said anode from yoke 13.

The permanent magnet 11 is connected with the magnetic yoke 13 through an intermediate insulating plate 21, at the left,'and thus is electrically insulated from the the rod cathode.

yoke. The soft-iron .pole shoe 16 carries rod cathode 22, the latter exending in axial relation to the cylindrical anode 15 and to the extraction opening 14. The diameter of the rod cathode 22 is small relative to the diameter of the cylinder anode 15. The rod cathode 22 terminates close to and in front of ion extraction opening 14. A cathode sheet 25 is provided centrally with an inwardly directed tubular projection 27, the cathode sheet being mounted by insulators 29, and being electrically connected by a lead 26 to terminal 24- through the cathode lead, thus being at the same potential .as the cathode 16. A diaphragm tube 28 projects from the extraction opening 14 into the discharge space. The extraction voltage is applied between the cathode surfaces 16a and 25 on the one hand the yoke '13 on the other hand.

FIG. 3 shows .a cold ion source through which the rod cathode 22 protrudes only to about the middle of the discharge space, within tubular anode 15. The inwardly directed tubular extension 27 of the diaphragm plate or cathode sheet 25 extends toward the middle of the discharge space, thus greatly promoting the elfectiveness of The rod cathode 22 is hollow, being provided with a feeder pipe (not numbered) which passes through the pot-shaped magnetizable yoke 13, in insulated relation thereto. Hollow cathode 22 is preferably a capillary tube. At its inlet the feeder tube carries a feeder funnel 31, and also a lock device 32 here shown in the form of two cock valves (not numbered). The interior head or end portion of the rod cathode 22 is perforated, to provide a screen or sieve, as shown in FIG. 4. The feeder device is employed to pass the substance to be investigated from the outside to the perforated end of the rod cathode, for instance by the action of or with the aid of the ambient air pressure.

FIG. 5 illustrates a cold ion source having readily exchangeable components. Parts of the ion source that may be contaminated, in the mass spectrographical sense, by the substance being investigated, are designed and arranged to be easily exchanged. A tube 33 envelopes the mass-filter measuring cell, comprising a removable iron yoke piece 35 mounted on a base plate 34, the base plate being readily removable from the envelope tube 33, together with the yoke 35. Yoke piece 35 carries rod cathode 22 and the entire sheet cathode 25, the latter being electrically and mechanically connected with the yoke piece. Anode 15 is disposed centrally about rod cathode 22 and centrally of and in the interior of cathode sheet 25. The anode lead extends from a terminal 17 and through an insulating bushing 18 in base plate 34 to the interior of envelope tube 33. Another lead extends from an external terminal 24 through an insulator 23, in the base plate 34, to the yoke piece 35, being thus connected to the cathode. The flange of envelope tube 33 is connected to another terminal 19 by which the envelope tube, yoke piece, and cathode can be grounded. If the envelope tube 33 is non-metallic, the component provided with the extraction diaphragm is to be grounded directly. The vessel is vacuum-tightly sealed by gaskets such as the ring gasket 38. Permanent magnets 11 provide the necessary magnetic flux. The magnetic circuit is closed through yoke piece 35 and the soft-iron plates 36 and 37, which are preferably displaceable. In all other respects the embodiment corresponds to those described with reference to FIGS. 2 and 3, as is apparent from the use of the same reference numerals for respectively similar components.

The various forms of cold ion source can be combined with each other as may be desired.

The general operation of the described mass filter is as described in the above-mentioned patent and applications. For particulars of the use of the equipment and method in the analysis of solid bodies, reference is to be made to the description of the performance found in the introductory part of this specification.

We claim:

1. An apparatus for separating ions having different specific charges; comprising an evacuated vessel; means including electrodes in said vessel for creating an electric field therein, said means supplying to said electrodes a Voltage having an arbitrary periodical function of time flt), creating a time-periodical field the potential of which is the general quadratic function of the rectangular coordinates x, y, z of the electrode arrangement, a, 18 and 'y being constants satisfying the equation ut+fi='y; cold-cathode electron-collision ion source means for creating charged ions in said evacuated vessel, the said ions being introduced into said field whereby due to the electrostatic forces of the field on the ions certain ions perform oscillations of a limited amplitude and others perform oscillations of an increasing amplitude, depending on their respective specific charges, and therefore follow stable and instable paths, respectively, and are thereby separated, said ion source means comprising a cathode structure having means providing an extraction channel through which ions may pass into the space where they are to be utilized, anode means coaxially disposed with respect to said extraction channel, said cathode structure comprising electric-field concentrating means having its maximum field density near said extraction channel at the anode side thereof a hollow rod cathode extending toward said extraction channel and magnet means having a magnetic pole end at said same location, whereby said electric and magnetic fields spacially and temporally fix the discharge plasma near said extraction channel, the material under investigation being disposed in said hollow rod cathode and being released at that end of said hollow rod cathode which is directed toward the analyzer part of the apparatus away from the said electron-collision ion source.

2. An apparatus as claimed in claim 1, wherein at least those parts of the ion source which can be contaminated by the substance to be investigated are readily removable, for ease of replacement.

3. An apparatus as claimed in claim 1, further comprising a feeder tube communicating with said hollow rod cathode whereby test specimen material can be introduced through the feeder tube from the outside of the measuring cell into the cathode.

4. An apparatus as claimed in claim 3, wherein said feeder tube is provided with a lock device in a section thereof, so that the test specimen can be isolated in said section of the tube, alternatively from communication with the outside, and with the inside of the measuring apparatus.

5. In an apparatus for separating ions having different specific charges, comprising an evacuated vessel; and means including electrodes in said vessel for creating an electric field therein, said means supplying to said electrodes a voltage having an arbitrary periodical function of time f(t); cold-cathode electron-collision ion source means for creating charged ions in said evacuated vessel, the said ions being introduced into said field whereby due to the electrostatic forces of the field on the ions certain ions perform oscillations of a limited amplitude and others perform oscillations of an increasing amplitude,

depending on their respective specific charges, and therefore follow stable and instable paths, respectively, and are thereby separated, said ion source means comprising a cathode structure having means providing an extraction channel through which ions may pass into the space where they are to be utilized, anode means coaxially disposed with respect to said extraction channel, said cathode structure comprising electric-field concentrating means having its maximum field density near said extraction channel at the anode side thereof, a hollow rod cathode extending toward said extraction channel, and magnet means having a magnetic pole end at said same location, whereby said electric and magnetic fields spacially and temporally fix the discharge plasma near said extraction channel, the material under investigation being disposed in said hollow rod cathode and being released at that end of said hollow rod cathode which is directed toward the analyzer part of the apparatus away from the said electron-collision ion source.

6. For an apparatus for separating ions having different specific charges a cold-cathode electron collision ion source comprising a cathode structure having means providing an extraction channel through which ions may pass into the space where they are to be utilized, and anode means, the cathode structure including at least one hollow rod cathode extending within the extraction channel, the material under investigation being disposed in said hollow rod cathode and being located at that end of said hollow rod cathode which is directed toward the analyzer part of the apparatus away from the said electron-collision ion source.

7. An electron collision ion source, comprising a cathode structure forming an extraction channel through which the ions may pass into the space where they are to be utilized, anode means coaxially disposed with respect to said extraction channel, said cathode structure comprising electric field concentrating means having its maximum field density near said extraction channel at the anode side thereof and including a hollow rod cathode extending toward said extraction channel, and magnet means having a magnetic pole end at the same location for combining with said electric field to temporally and spacially fix the discharge plasma near said extraction channel, material to be investigated being disposed in said hollow rod cathode.

References Cited by the Examiner UNITED STATES PATENTS 2,622,204 12/1952 Shaw et al. 250-419 2,852,683 9/1958 Peters et al 250-419 2,934,665 4/1960 Ziegler 250-419 2,939,952 6/1960 Paul et al 250 41.9 2,941,099 6/1960 Picard et a1 313-231 X 3,018,399 1/1962 Shelton 313-230 X OTHER REFERENCES Pulsed High-Intensity Ion Source, by Rolf Pauli and Jan Flinta, Nuclear Instruments 2, North Holland Publishing 00., Amsterdam, 1958, Part I, pages 219-226; Part II, pages 227236.

RALPH G. NILSON, Primary Examiner. 

1. AN APPARATUS FOR SEPARATING IONS HAVING DIFFERENT SPECIFIC CHARGES; COMPRISING AN EVACUATED VESSEL; MEANS INCLUDING ELECTRODES IN SAID VESSEL FOR CREATING AN ELECTRIC FIELD THEREIN, SAID MEANS SUPPLYING TO SAID ELECTRODES A VOLTAGE HAVING AN ARBITRARY PERIODICAL FUNCTION OF TIME F(T), CREATING A TIME-PERIODICAL FIELD THE POTENTIAL OF WHICH IS THE GENERAL QUADRATIC FUNCTION 